1. Field of the Invention
The present invention relates to an optical-type zoom lens barrel called an internal focusing or rear focusing type barrel.
2. Related Background Art
In addition to use of compact zoom lens barrels of higher magnification, there is a general trend toward automatic focusing in recent years. Demand has been increasing for an internal focusing or rear focusing type barrel in which focus lenses are lightweight and movable components are not in direct contact with the outside. Due to such an increase in demand, various mechanisms described below have been proposed.
A zoom lens barrel has been proposed in Japanese Patent Laid-Open Nos. 57-37307, 63-304217, 64-35515, 64-35516, etc. Such a zoom lens barrel is designed so that tracks along which a focus lens group is moved while zooming is being performed functions in a manner related to a part of a focusing operation performed by the focus lens group. (This type of zoom lens barrel is referred to as a one-cam type barrel.) In addition to this type of barrel, there are additional types of zoom lens barrels described below.
(A) A zoom lens barrel disclosed in U.S. patent application Ser. No. 07/603,539 has been proposed by the applicant of this invention. It is designed so that tracks along with a focus lens group is moved while zooming is being carried out functions independently of a focusing operation performed by the focus lens groups. An independent focusing cam is provided for such a zoom lens barrel. (This type of zoom lens barrel is referred to as a two-cam type barrel.)
(B) A zoom lens barrel of a two-cam type is disclosed in Japanese Patent Laid-Open No. 63-49715.
(C) A zoom lens barrel in which a first lens group is disposed in a fixing portion is disclosed in Japanese Patent Application Laid-Open No. 63-314511.
The conventional zoom lens barrels mentioned above have the following disadvantages:
As regards (A), a cam member for focusing and another cam member for a zoom lens group are required, thus increasing the cost thereof.
As regards (B), it has more components than the two-cam type barrel has.
As regards (C), because the first lens group is a fixed lens group, it is difficult to render lenses compact.
To change the closest camera-to-object distance by zooming, a mechanical stopper, for changing the angle through which a focus lens rotates, has been used for a long time, and a modification has been made to a shape on the closest end of a lens barrel.
As mentioned previously, there has been a general trend toward automatic focusing for zoom lenses, and demand has been increasing for the internal focusing or rear focusing type barrel in which focus lenses are lightweight and movable components are not in direct contact with the outside. With a zoom lens having 3-4 times magnification, even if the closest distance in a telephoto state restricts the closest distance in a wide state, this does not cause much trouble. The same closest distance can be used over an entire zooming range.
However, if a zoom lens of high magnification is used, it is difficult to satisfy optical properties in the telephoto state at the closest distance. For this reason, even when optical properties in the wide state at the closest distance are excellent, a photograph can be taken only within the closest camera-to-object distance which is determined by the optical properties in the telephoto state.
As regards automatic focusing, a switching mechanism is provided in which a rotation angle for focusing restricts the closest distance for focusing. Because zooming is carried out by hand, when the switching mechanism is charged, an over current flows to a focus driving motor. Consideration must be taken to prevent such a trouble from occurring.
The following structures have hitherto been used for retaining a first lens barrel which moves a first lens group a considerable distance toward an object. (1) Either a fixed barrel having a mating portion of a predetermined length or a cam barrel rotatable at a fixed position is used to retain the first lens barrel. (2) The first lens barrel is provided with a plurality of cam followers which are engaged with cam grooves formed on a cam barrel rotatable at a fixed position and with straight grooves formed on a fixed lens barrel. The first lens barrel is thereby prevented from collapsing.
As regards item (1), however, it is required that the length of the first lens barrel be approximately equal to the radius of the mating portion besides the distance the first lens group moves. This becomes an obstacle to rendering the zoom lenses compact.
As regards item (2), it is necessary to render the cam followers rigid. Errors in the collapse of the lens barrel increase, depending on the rising angle of a cam.